Abstract:

Davidson Seamount, a volcano located about 80 km off the central California coast, has a volume of
~320 km³ and consists of a series of parallel ridges serrated with steep cones. Davidson was sampled and
its morphology observed during 27 ROV Tiburon dives. During those dives, 286 samples of lava,
volcaniclastite, and erratics from the continental margin were collected, with additional samples from one
ROV-collected push core and four gravity cores. We report glass compositions for 99 samples and
⁴⁰Ar-³⁹Ar incremental heating age data for 20 of the samples. The glass analyses are of hawaiite (62%),
mugearite (13%), alkalic basalt (9%), and tephrite (8%), with minor transitional basalt (2%), benmoreite
(2%), and trachyandesite (2%). The lithologies are irregularly distributed in space and time. The volcano
erupted onto crust inferred to be 20 Ma from seafloor magnetic anomalies. Ages of the lavas range from
9.8 to 14.8 Ma. The oldest rocks are from the central ridge, and the youngest are from the flanks and
southern end of the edifice. The compositions of the 18 reliably dated volcanic cones vary with age such
that the oldest lavas are the most fractionated. The melts lost 65% to nearly 95% of their initial S because
of bubble loss during vesiculation, and the shallowest samples have S contents similar to lava erupted
subaerially in Hawaii. Despite this similarity in S contents, there is scant other evidence to suggest that
Davidson was ever an island. The numerous small cones of disparate chemistry and the long eruptive
period suggest episodic growth of the volcano over at least 5 Myr and perhaps as long as 10 Myr if it began
to grow when the spreading ridge was abandoned.